Manufacture of multilayer film containing very low density polyethylene

ABSTRACT

A coextruded, heat shrinkable, thermoplastic multilayer film for packaging fresh red meat cuts and processed meats. The film is produced by coextruding a multilayer structure which includes a first layer comprising very low density polyethylene, a first core layer of vinylidene chloride-methyl acrylate copolymer, and a second layer comprising very low density polyethylene. In another embodiment the first layer is a second core layer confined between said first core layer and a first outer layer comprising a heat sealable thermoplastic polymer or copolymer. In a further embodiment the second layer is a second core layer confined between said first core layer and a second outer layer comprising a thermoplastic polymer or copolymer.

RELATED CASES

This is a continuation of application Ser. No. 07/263,947 filed Oct. 28,1988, now abandoned, which is a continuation of application Ser. No.07/054,918 filed May 28, 1987 and issued as U.S. Pat. No. 4,863,784.

FIELD OF THE INVENTION

This invention relates to a coextruded, heat shrinkable, thermoplasticmultilayer film suitable for use in the manufacture of bags forpackaging fresh red meats and processed meats. In particular, thisinvention relates to a coextruded heat shrinkable multilayer film havinga first outer layer of very low density polyethylene, a process ofproducing a core layer of vinylidene chloride-methyl acrylate copolymer,and a second outer layer of very low density polyethylene.

BACKGROUND OF THE INVENTION

The Meat Packaging Industry may be commonly divided into three segments.They are fresh meats, frozen meats and processed meats. This inventionrelates to fresh red meats, such as beef and pork, which is distinctfrom fresh white meat, such as poultry. This invention also relates toprocessed meats.

In the Fresh Red Meat Industry, the cattle and swine are slaughtered andbroken down into primal and subprimal meat cuts. The primal andsubprimal meat cuts are large cuts of meat. They are smaller than a sideof beef, for example, but larger than the ultimate cut which is sold atretail to the consumer. A primal cut comprises the entire section of aside beef, such as the rib section or the rump roast section, while asubprimal cut comprises only a portion of such a section. Primal andsubprimal cuts are prepared at the slaughter house and are then shippedto a retail meat store, or to an institution such as a hospital, hotelor restaurant, where they are butchered into small cuts of meat suitablefor the individual consumer.

The Processed Meat Industry takes various portions of the animalcarcasses and processes these portions under varying conditions toproduce finished meat products which may be used directly by theconsumer. Products may include ham, smoked picnics, smoked butts, cornedbeef, turkey breast, and various sausage products such as frankfurters,smoked sausage links, bologna, salami, and the like. These products maybe package in consumer portions or they may be packaged in bulk forshipment to a retail meat store, restaurant or hotel. Bulk shipments mayinclude such items as ham chunks, cooked turkey breasts, bologna chubs,long bologna for delicatessen sale, rings of bologna, corned beefbrisket, smoked picnics, smoked butts and linked products such as smokedsausage.

When fresh red meat cuts, such as roast or rib sections, and bulkprocessed meats are prepared for shipment or storage, they are usuallypackaged in such a way that air (i.e., oxygen) is prevented fromcontacting the meat and moisture is prevented from leaving the meat.This is done in order to minimize spoilage and discoloration duringshipping and handling. One desirable way to package fresh red meats andprocessed meats so as to protect them from contact with air and frommoisture loss is to shrink package them with a packaging material thathas good oxygen and moisture vapor barrier properties. One such shrinkpackaging material that has good oxygen and moisture vapor barrierproperties is polyvinylidene chloride film. Vinylidene chloride-vinylchloride copolymers are commonly referred to as PVDC.

While vinylidene chloride-vinyl chloride copolymer film has excellentbarrier properties, in actual practice, when PVDC is used as a monolayerfilm, it must be plasticized in order for the film to have adequateabrasion resistance and flexibility at storage temperature of, forexample, 30° to 50° F. Unfortunately, the addition of plasticizersufficient to provide the requisite low temperature properties to thePVDC monolayer film has a significant adverse effect on the barrierproperties of the film. While increasing the thickness of the film fromthe conventional thickness of 1.5 to 2.0 mils, to 5 mils or more, forinstance, would improve the barrier properties of the film, it would beeconomically undesirable to use a monolayer film of PVDC having athickness of 5 or more mils. Also, if such thick films were employed,bags made from the film would be difficult to gather and clip closed atthe open end.

One approach to the provision of a film having barrier properties whichare better than those of the 1.5 to 2.0 mil monolayer PVDC filmpreviously used for shrink packaging meat, is to employ a multilayerfilm, one layer of which is vinylidene chloride-vinyl chloride copolymerhaving a minimum amount of plasticizer. The other layer or layers ofsuch multilayer films are selected so as to provide the requisite lowtemperature properties and abrasion resistance which are lacking in thevinylidene chloride-vinyl chloride layer containing little or noplasticizer.

In providing such a film, however, it must be recognized that goodoxygen and moisture vapor barrier properties, abrasion resistance, andlow temperature properties are not the only requirements for a film thatis to be used for shrink packaging processed meats and primal andsubprimal meat cuts. The film must have been biaxially stretched inorder to produce shrinkage characteristics sufficient for the film toheat-shrink within a specified range of percentages, e.g., from about 15to 60 percent at about 90° C., in both the machine and the transversedirections. (Conventionally, the term "MD" refers to machine directionand the term "TD" refers to transverse direction.) The film must be heatsealable in order to be able to fabricate bags from the film and inorder to heat seal the open ends of the fabricated bags after insertionof the meat product. The heat sealed seams of the bags must not pullapart during the heat shrinking operation, and the film must resistpuncturing by sharp bone edges during the heat shrinking operation.

Also, there must be adequate adhesion between the several layers of thefilm so that delamination does not occur, either during the heatshrinking operation or during exposure of the film to the relativelyhigh temperatures that may be reached during shipping and storage of thefilm in the summertime. Delamination is the phenomenon where layers ofthe multilayer film are readily separable or easily pulled apart fromface to face integrity with no tearing of the individual layers of film.

In order to overcome problems of delamination, it is known in the priorart to use adhesive layers between layers which do not otherwise adhereto each other with the required bonding strength under normal conditionsof commercial use. For example, polyethylene and polypropylene are knownto have poor lamination characteristics when in face to facerelationship with the oxygen barrier layers conventionally used in bagsof multilayer films used for packaging primal or subprimal meat cuts offresh red meat, and in bags for packaging processed meats. Adhesivelayers are often used in compensation for such poor laminationcharacteristics.

By way of illustrating multilayer film containing adhesive layers,Shirmer U.S. Pat. No. 4,448,792 teaches a cook-in shrink bag fabricatedfrom a multilayer film having a first meat sealing and food contactinglayer composed of propylene homopolymer or copolymer; a second heatshrinkable layer composed of a blend or propylene homopolymer orcopolymer and butylene homopolymer or copolymer; a third adhesive layercomposed of irradiatively cross-linkable ethylene copolymer; a fourthoxygen barrier layer comprising vinylidene chloride copolymer; a fifthadhesive layer of irradiatively cross-linkable ethylene copolymer; and asixth optical clarity layer comprising propylene homopolymer orcopolymer; wherein the entire six layer film has been irradiated eitherbefore or after the multilayer film has been biaxially stretched. Amongthe several key benefits enumerated by Schirmer, the completelyirradiated six layer film provides bags having structural integrity inthat the bags resist delamination and their heat seals have hightemperature resistance, while at the same time the bags are acceptablefor food contact in terms of minimum levels of extractable. Cook-in bagsmade of this multilayer film will maintain seal integrity and willresist delamination when submerged in water at 80° C. for 12 hours.

While the use of adhesive layers, as taught in such prior art, iseffective in avoiding problems of delamination, it is an undesirablesolution. First of all, additional equipment is required, and theprocess becomes more complex with the need for new extruders andextrusion dies. Additionally, the addition of adhesive layers willgenerally make the film thicker. The production of a thicker film may beavoided, however, by reducing the thickness of the other film layers orby controlling the thickness of the adhesive layers so that they arevery thin. Since the oxygen barrier layer thickness may not be reducedwithout the loss of barrier effectiveness, it becomes necessary to onlyreduce the thickness of the outer layers, and this can cause a reductionin heat sealability and/or a reduction in puncture resistance. Thus, itis preferred not to reduce the thickness of the outer film layers, butto control the adhesive layers to a minimum thickness which is effectivein bonding the layers sufficiently to avoid delamination problems. Thisin turn requires sophisticated equipment, which is expensive, since theequipment must control the adhesive layer thickness generally to a rangeof from about 0.10 to about 0.15 mil. Moreover, the adhesives themselvesare generally very expensive, and the cost of adhesive plus the cost ofthe new equipment generally causes an increase in the price of the bagsproduced from the multilayer film.

Thus, it is preferred to fine a means for strongly bonding the otherlayers directly to the oxygen barrier layer without the use of prior artadhesive layers.

It should be noted that the aforementioned Shirmer U.S. Pat. No.4,448,792 teaches that both polypropylene and blends of propylene withanother polymer require an adhesive layer in order to bond these layersacceptable to the oxygen barrier layer of vinylidene chloride copolymer.However, the prior art also teaches such a need for adhesive layers inregard to polyethylene.

Illustrative of a polyethylene, unblended with any other polymer, whichrequires an adhesive layer in order to acceptable bond to the oxygenbarrier layer is U.S. Pat. No. 4,640,856 to Ferguson et.al. This patentdiscloses bags for the packaging of fresh red meat (primal and subprimalmeat cuts), cheeses, poultry and other food and non-food products,wherein the bag is fabricated from a multilayer thermoplastic shrinkfilm having a substrate layer of a very low density polyethylene whichhas been extrusion coated with at lest a gas barrier layer comprising acopolymer of vinylidene chloride or a hydrolized ethylene vinyl acetatecopolymer. This two layer film is then extrusion coated with anotherlayer of a thermoplastic polymer to form at least a three layer filmhaving a core layer of the gas barrier layer. Ferguson et.al. teach thatan ethylene vinyl acetate layer should be interposed between the surfacelayer of very low density polyethylene and the gas barrier core layer ofvinylidene chloride copolymer to promote adhesion between the layers andto lessen any tendency of the film to delaminate, since very low densitypolyethylene does not adhere to vinylidene chloride copolymers as wellas the ethylene vinyl acetate does. Additionally, this patent teachesthat for maximum delamination protection adhesion layers should be usedto bond outer layers of very low density polyethylene to core layers ofbarrier film, both when the oxygen barrier layer is a copolymer ofvinylidene chloride and when it is a hydrolyzed ethylene vinyl acetate.

Illustrative of a polyethylene blended with another polymer in one layerof a multilayer film, wherein an adhesion layer is required in order toacceptably bond the blend layer to the oxygen barrier layer, is U.S.Pat. No. 4,456,646 to Nishimoto et.al. This patent teaches that amultilayer film for the packaging of meats and cheeses may be fabricatedof a core layer of a vinylidene chloride copolymer with outer layers ofa blend of ethylene vinyl acetate and a linear low density polyethylenehaving a density of from 0.900 to 0.950 grams per cubic centimeter.Nishimoto et.al. teach that the linear low density polyethylene, whichis a copolymer of ethylene with an alpha olefin having less than 18carbon atoms, will not adhere to the core layer of vinylidene chloridecopolymer so that the outer layers are apt to delaminate from the corelayers, particularly in an elevated temperature environment.Accordingly, Nishimoto et.al. teach that it is necessary to provide anadhesive layer between the core layer of barrier film and each outerlayer containing the blend of ethylene vinyl acetate and linear lowdensity polyethylene.

We have found that not only are the multilayer film structures ofFerguson et.al. U.S. Pat. No. 4,640,856 undesirable from the standpointof the adhesive layer requirements, but in fact, contrary to theteachings of Ferguson et.al., we have discovered that a first outerlayer of very low density polyethylene will bond directly to a corelayer of oxygen barrier film comprising a vinylidene chloride copolymerwithout the use of any adhesive layer interposed therebetween, if themultilayer film is a coextruded film. This matter will be discussed morefully hereinafter.

We have also found that not only are the polyethylene blends ofNishimoto et.al. U.S. Pat. No. 4,456,646 undesirable from the standpointof the adhesive layer requirement, but they are also undesirable becausethey cause the optical properties of the film to be unacceptabledegraded. Such polyethylene blends cause the haze value for themultilayer film to increase to an unacceptable level, and they cause thegloss value for the multilayer film to decrease to an unacceptablelevel.

The haze value is important because it is an indication of the abilityof the film to transmit light. A low haze value indicates a very clearfilm which enables one to clearly see the contents of the package. Wefind that the haze value must not exceed 6.5% when packaging fresh redmeat cuts and processed meats. This value is particularly important inregard to the packaging of processed meats, since it is the individualconsumer who is viewing the package and deciding whether or not to makea purchase.

The gloss value is important because it is a measure of the shinyappearance of the film. A high gloss value indicates that the packagedmeat product will have a very shiny highly attractive appearance. Wefind that the gloss value should not be below 70 % when packaging freshred meat cuts and processed meats. This value is particularly importantin regard to the packaging of processed meats, since it is theindividual consumer who is viewing the package at the point of purchase.

Another important consideration in evaluating multilayer films is thetype and degree of film curl which the film exhibits. Film curl is anindication of the ease or the difficulty which is experienced in openinga bag which has been fabricated from the multilayer film. In order for amultilayer film to be commercially acceptable, the film must be capableof producing bags which are easily opened by the bagging operator whoplaces the meat product into the bag on the production line of the meatpacking plant. Bags which are difficult to open cause delays in thebagging operation and result in low production efficient. Thesignificance of film curl will be discussed more fully hereinafter.

In summary then, it is an object of the present invention to provide amultilayer film containing a core layer of an oxygen barrier and outerlayers of polyethylene, wherein the outer layers are bonded directly tothe core layer with no adhesive layers interposed therebetween.

It is another object of the present invention to provide such multilayerfilms, wherein the haze and gloss properties of the multilayer film areacceptable under conditions of commercial use.

It is a further object of the present invention to provide suchmultilayer films, wherein the films are capable of fabrication into bagswhich are easily openable under conditions of commercial use, asindicated by the curl properties of such multilayer films.

It is a still further object of the present invention to provide suchmultilayer films, wherein no adhesive layers are employed to bond theouter layers to the core layer, and wherein the outer layers provideimproved strength characteristics and improved puncture resistance tothe film.

SUMMARY OF THE INVENTION

The foregoing objectives of the present invention may be achieved byproviding a coextruded, thermoplastic, heat shrinkable, multilayer filmwherein, (a) said multilayer film comprises a first layer comprisingvery low density polyethylene, a core layer comprising vinylidenechloride-methyl acrylate copolymer, and a second layer comprising verylow density polyethylene; and wherein (b) said first layer is adhereddirectly to one side of said core layer and said second layer is adhereddirectly to the other side of said core layer.

In one preferred embodiment, the present invention provides thismultilayer film wherein the first and second layers of very low densitypolyethylene are outer layers, and the core layer is confined betweenthese outer layers to provide a three layer film.

In another preferred embodiment, the core layer of vinylidenechloride-methyl acrylate is a first core layer, and either the firstlayer or the second layer of very low density polyethylene is a secondcore layer confined between the first core layer and an outer layer ofthermoplastic polymer or copolymer to provide a multilayer film havingfour or more layers.

In a further preferred embodiment, both the first layer and the secondlayer of very low density polyethylene are core layers confined betweenthe core layer of vinylidene chloride-methyl acrylate copolymer and twoouter layers of thermoplastic polymer of copolymer to provide amultilayer film having five or more layers.

Despite the foregoing teachings of the prior art, we have now discoveredthat multilayer films having a core layer of an oxygen barrier ofvinylidene chloride-methyl acrylate copolymer may contain layers of verylow density polyethylene which are bonded directly to the core layerwithout the use of interposed adhesive layers. Further, we havediscovered that the multilayer films of this invention will not haveincreased haze or diminished gloss in comparison to prior art multilayerfilms currently in commercial use. Additionally, we have found that themultilayer films of this invention have acceptable film curl, and thatthey have improved tensile strength and improved puncture resistance.

The multilayer films of the present invention may be furthercharacterized by the fact that the first layer of very low densitypolyethylene and the second layer of very low density polyethylene willbond directly to the two sides of the core layer of vinylidenechloride-methyl acrylate copolymer even though said first layer and saidsecond layer both have substantial freedom from cross-linking bonds.However, it is also within the scope of the present invention for saidfirst layer and said second layer to comprise very low densitypolyethylene which contains cross-linking bonds.

DETAILED DESCRIPTION

All embodiments of the present invention comprise a coextrudedmultilayer film, suitable for use in packaging fresh red meat cuts andprocessed meats, which comprises a core layer of vinylidenechloride-methyl acrylate copolymer, a first layer comprising very lowdensity polyethylene, and a second layer comprising very low densitypolyethylene, wherein the first layer and the second layer are bondeddirectly to the first and second surfaces of the core layer without theuse of adhesive layers.

It must be emphasized at this point that our invention is directed tocoextruded multilayer films, because we have discovered that highadhesion bonding of very low density polyethylene directly to the corelayer of vinylidene chloride-methyl acrylate copolymer, without the useof interposed adhesive layers, can be achieved through coextrusion. Thisis because the coextruded multilayer film is produced by joining theseveral layers together while all layers are in the liquid phase. Thisallows the various polymers at the layer-to-layer interface of liquid tointermingle slightly so that when the liquids solidify, the layers arestrongly bonded to each other.

This explains why U.S. Pat. No. 4,640,856 to Ferguson et.al. teachesthat adhesive layers are required in order to bond very low densitypolyethylene to the vinylidene chloride copolymer. The experimental runswhich produce the multilayer film samples for the Examples in thisPatent, wherein the oxygen barrier layer was a copolymer of vinylidenechloride, were all produced by the extrusion coating process which isexemplified by U.S. Pat. No. 3,741,253 to Brax et.al. In this extrusioncoating process, a substrate layer is first extruded to provide atubular film which will become the inner layer of the tubular multilayerfilm. After this base layer has been made, a melt of the oxygen barrierlayer comprising the copolymer of vinyldene chloride is extrusion coatedon the outer surface of the tubular film. After this extrusion coatinghas solidified to provide a two layer substrate tubular film, a thirdlayer of a melted polymer of copolymer is coated on the outer surface ofthe two layer tubular film to provide a three layer tubular film.Succeeding layers of other polymers or copolymers may be extruded onthis three layer tubular film to the extent that tubular multilayerfilms containing more than three layers are desired. It is due to thefact that the succeeding polymer melts are coated upon a solid filmsubstrate in each instance, that causes the succeeding layers to havevery poor bonding when very low density polyethylene and vinylidenechloride copolymers are extrusion coated to one another. The polymermelt is unable to penetrate the solid surface of the film substratesufficiently to allow the two layers to slightly intermingle at thelayer-to-layer interface and thereby produce a strong interface bondingwhen the melted coating solidifies.

In all embodiments of the present invention, the core layer of oxygenbarrier film comprises vinylidene chloride-methyl acrylate copolymer.The vinylidene chloride content of the copolymer preferably should notexceed about 95 weight percent. This is because, when the vinylidenechloride content is greater than about 95 weight percent, the vinylidenechloride-methyl acrylate copolymer is generally not extrudable inpresently known coextrusion systems. However, the vinylidene chloridecontent preferably should not be less than about 85 weight percent ofthe vinylidene chloride-methyl acrylate copolymer in order to maintainthe level of methyl acrylate in the copolymer at not greater than 15weight percent, which is the maximum level of methyl acrylate currentlyallowed by the United States Food and Drug Administration for foodcontact applications.

It is within the scope of the present invention for the core layer ofoxygen barrier film to comprise vinylidene chloride-methyl acrylatecopolymer. It is also within the scope of the present invention for theoxygen barrier core layer to contain vinylidene chloride-methyl acrylatecopolymer blended with another oxygen barrier material, such asvinylidene chloride-vinyl chloride copolymer. It should be noted,however, that multilayer films containing blends of vinylidenechloride-methyl acrylate copolymer with vinylidene chloride-vinylchloride copolymer, and having adjacent layers of thermoplastic polymersin general, are not a part of this invention, but are a separateinvention which is claimed in copending application Ser. No. (D-20017)filed contemporaneously with this application in the name of J. M.Schuetz.

The vinylidene chloride-vinyl chloride copolymer, which may be blendedwith the vinylidene chloride-methyl acrylate copolymer, will contain atleast about 65 weight percent, and not more than about 95 weightpercent, of polymerized vinylidene chloride because, when the vinylidenechloride content is less than about 65 weight percent, the oxygen andmoisture barrier property of the copolymer is diminished. If thevinylidene chloride content is more than 95 weight percent, thevinylidene chloride-vinyl chloride copolymer is generally notextrudable.

The vinylidene chloride-methyl acrylate copolymer and the vinylidenechloride-vinyl chloride copolymer each will preferably contain less than5 weight percent of a plasticizer, the percentages being based on theweight of the total blend, i.e., including the copolymer and alladditives such as the plasticizer, in order to maximize the barrierproperties of the film. Conventional plasticizers such as dibutylsebacate and expodized soybean oil may be employed therein.

All embodiments of the present invention contain a first layercomprising a very low density polyethylene and a second layer comprisingvery low density polyethylene. Those skilled in the art recognize thatthis is a specific species of polyethylene. Several species ofpolyethylene are commercially available, and these species may becharacterized as the product of a high pressure catalytic process or theproduct of a low pressure catalytic process.

The high pressure process produces polymers which are highly branched,with higher densities being an indication of shorter branches and highcrystallinity. Such polymers are conventionally classified as lowdensity polyethylene, commonly called "LDPE", which has a density belowabout 0.925 grams per cubic centimeter, and high density polyethylene,commonly called "HDPE", which has a density greater than about 0.940grams per cubic centimeter. Polyethylenes having a density in the rangeof from about 0.925 to about 0.940 grams per cubic centimeter arecommonly referred to as medium density polyethylene.

The ethylene may also be polymerized in the high pressure process withother monomers, such as vinyl acetate, ethyl acrylate, or acrylic acid.The copolymer with vinyl acetate is known as ethylene vinyl acetate andit is commonly referred to as "EVA".

The low pressure process produces polymers which are more linear instructure. Such polymers are commonly classified as very low densitypolyethylene, commonly called "VLDPE", which has a density of from about0.860 to about 0.915 grams per cubic centimeter, and linear low densitypolyethylene, commonly called "LLDPE", which has a density greater thanabout 0.915 grams per cubic centimeter.

Very low density polyethylene and linear low density polyethylene arecopolymers of ethylene with a higher alpha olefin. The higher alphaolefins which can be polymerized with ethylene to produce the lowmodulus linear copolymers can contain from three to eight carbon atoms.These alpha olefins should not contain any branching on any of theircarbon atoms closer than two carbon atoms removed from the double bond.Suitable alpha olefins include propylene, butene-1, pentene-1, hexene-1,4-methylpentene-1, heptene-1 and octene-1. The preferred alpha olefinsare propylene, butene-1, hexene-1, 4-methylpentene-1 and octene-1.

In some instances, one or more dienes, either conjugated ornon-conjugated, may have been present in the polymerization reactionmixture. Such dienes may include, for example, butadiene, 1,4-hexadiene,1,5-hexadiene, vinyl norbornene, ethylidene norbornene anddicyclopentadiene.

The linear polyethylene produced by this low pressure catalyticcopolymerization, typically has a melt index of from about 0.5 to about2.5 decigrams per minute. When the melt index is below 0.5 decigrams perminute, the film is difficult to extrude, and resins having a melt indexabout 2.5 decigrams per minute are not film grade resins.

As previously noted hereinabove, those copolymers having a density inthe range of from about 0.86 to about 0.915 grams per cubic centimeterare commonly referred to as a very low density polyethylene, while thosehaving a density greater than about 0.915 grams per cubic centimeter arecommonly referred to as linear low density polyethylene.

In one preferred embodiment of the present invention, the first layer ofvery low density polyethylene, which is bonded directly to the oxygenbarrier core layer of vinylidene chloride-methyl acrylate copolymer,provides the heat sealing layer for the multilayer film. Where the filmis produced by coextrusion to provide a tubular multilayer film, thefirst layer of very low density polyethylene will be the inner layer ofthe tubular film.

In other preferred embodiment of the present invention, the second layerof very low density polyethylene, which is bonded directly to the corelayer of vinylidene chloride-methyl acrylate copolymer without the useof adhesives, provides the second outer layer of a three layer filmembodiment. It has been found that using a very low density polyethylenein the second outer layer of the multilayer film provides the film witha puncture resistance which is improved over similar films containingouter layers of ethylene-vinyl acetate copolymers, and it also providesloading and shrink tunnel survival rates which are at least equal to, ifnot superior to, the survival rates of those films which containethylene vinyl acetate copolymer outer layers.

In an alternate embodiment of the present invention, however, the firstlayer of very low density polyethylene, which is bonded directly to theoxygen barrier core layer of vinylidene chloride-methyl acrylatecopolymer, provides a second core layer which is confined between thefirst core layer of oxygen barrier film and a first outer layer of aheat sealable thermoplastic polymer or copolymer. The heat sealablefirst outer layer may comprise such films as an ionomer, an ethylenevinyl acetate copolymer, an ethylene-propylene copolymer, and the like.Also suitable is polypropylene blended with another polymer, such aspolybutene-1. Thus, this embodiment contemplates a multilayer filmhaving four layers, although more than four layers are also possible.

In another alternate embodiment of the present invention, the secondlayer of very low density polyethylene, which is bonded directly to theoxygen barrier first core layer of vinylidene chloride-methyl acrylatecopolymer without the use of adhesives, provides a second core layerwhich is confined between the first core layer and a second outer layerwhich comprises a thermoplastic polymer or copolymer. Thus, thisembodiment of the present invention also contemplates another multilayerfilm having four layers, although multilayer films having more than fourlayers are also possible.

The addition of this fourth layer as a second outer layer for themultilayer film of this invention may be undertaken when it is desiredto improve the abrasion resistance of the film. This can be accomplishedby providing a second outer layer comprising a thermoplastic polymer orcopolymer such as an ionomer resin, a propyleneethylene copolymer, ahigh density polyethylene, a linear low density polyethylene, and thelike, as well as blends thereof.

In one preferred embodiment, the second outer layer (the fourth layer)comprises a blend of a high density polyethylene with a linear lowdensity polyethylene or a very low density polyethylene. As the level ofhigh density polyethylene in the blend of the second outer layer isincreased, the abrasion resistance of the film continues to increase.However, when the level of high density polyethylene in the second outerlayer blend becomes greater than about 5 weight percent, the haze valueof the film becomes unacceptable for bags used in the packaging ofprocessed meats. Additionally, when the level of high densitypolyethylene is increased to greater than 30 weight percent of thesecond outer layer blend, the shrinkage property of the film becomesunacceptable for bags used in the packaging of fresh red meats andprocessed meats. Thus, the amount of high density polyethylene in theblend should not exceed 30 weight percent in fresh red meat bags or 5weight percent in processed meat bags. The high density polyethyleneuseful in the second outer layer has a melt index of from about 0.1 toabout 1.0 decigram per minute, and a density of from about 0.94 to about0.96 gram per cubic centimeter. Resins having a melt index below 0.1 arenot extrudable, and those having a melt index above 1.0 produce films ofdiminished strength.

In summary then, the broadest aspect of the multilayer film of thisinvention contemplates a three layer film. However, it is contemplatedthat one or more layers may be added to the outer surface of either thefirst layer or the second layer of very low density polyethylene toprovide a multilayer film containing four or more layers. Moreover, itis also contemplates that one or more layers may be added to the outersurface of both the first layer and the second layer of very low densitypolyethylene to provide a multilayer film containing five or morelayers.

The coextruded thermoplastic multilayer films of this invention can beproduced by known techniques. For example, the multilayer films may beprepared by coextruding the film layers through an annular die toproduce a primary tube, and then biaxially stretching the multilayertubular film in accordance with the conventional "double-bubble"technique disclosed in Pahlke U.S. Pat. No. 3,456,044. Alternatively,the coextruded multilayer film may be slot cast and biaxially stretchedby tentering before the resulting sheet is fabricated into bags.

When the coextruded, multilayer film of the present invention has beenproduced, it may be desirable to cross-link the multilayer film,although cross-linking is not required in order to achieve high adhesionbonding of the very low density polyethylene directly to the vinylidenechloride-methyl acrylate core layer without the use of adhesive layersthereinbetween. Cross-linking may be undertaken in order to enhance theheat sealing characteristics of the first outer layer of very lowdensity polyethylene, which is the inner layer of the tubular filmembodiments. Cross-linking may also be undertaken in order to improvethe puncture resistance of the multilayer film. While chemicalcross-linking is feasible, we prefer to cross-link by irradiation.Chemical cross-linking may be achieved by means such as organic peroxidecross-linking, or by the addition of a silane to the very low densitypolyethylene and the subsequent reaction of the modified polyethylenewith a silanol condensation catalyst and water. We prefer to cross-linkby irradiation, since this technique is less complicated and it entailsa lower cost than the chemical cross-linking methods. Although theirradiation may be undertaken prior to the biaxial stretching step, weprefer to irradiate the multilayer film after biaxially stretching thefilm. The film is preferably irradiated with electrons at a dosage offrom about 1 to about 5 megarads, and more preferably at a dosage offrom about 2 to about 3 megarads.

In summary then, the multilayer films of the present invention may becharacterized by the fact that the first layer of very low densitypolyethylene and the second layer of very low density polyethylene willbond directly to the two sides of the first core layer of vinylidenechloride-methyl acrylate copolymer even though said first layer and saidsecond layer both have substantial freedom from cross-linking bonds.However, it is also within the scope of the present invention for saidfirst layer and said second layer to comprise very low densitypolyethylene which contains cross-linking bonds. Those skilled in theart recognize that cross-linking, whether by irradiation or by chemicalmeans, will cause the melt index of the cross-linked VLDPE layers to bebelow the melt index of the original VLDPE resins which are usedrespectively in the first layer of VLDPE and in the second layer ofVLDPE. Thus, the term "contains cross-linking bonds" means that the meltindex of the VLDPE layers is significantly lowered during the process ofconverting the respective resins of the two VLDPE layers into themultilayer film products of this invention. Similarly, the term "havingsubstantial freedom from cross-linking bonds" means that the melt indexof the VLDPE layers is not significantly lowered during the process ofconverting the respective resins of the two VLDPE layers into themultilayer film products of this invention.

In one preferred embodiment of the present invention, the coextrudedmultilayer film comprises a biaxially stretched thermoplastic threelayer film having a total thickness of from about 1.75 mils to about 4.5mils, and preferably from about 2.0 mils to about 3.0 mils. Films ofless than about 1.75 mils thickness will generally not have thenecessary puncture resistance, and films of greater than about 4.5 milswill produce bags which will be somewhat difficult to gather and clipclosed or they may be difficult to heat seal closed at efficient speedson heat sealing vacuum packing machines. The heat sealing first outerlayer of very low density polyethylene will preferably have a thicknessof from about 1.1 mils to about 1.8 mils; the core layer of vinylidenechloride-methyl acrylate copolymer will preferably have a thickness offrom about 0.25 mil to about 0.45 mil; and the second outer layer ofvery low density polyethylene will have a thickness of from about 0.35mil to about 2.0 mils, but preferably from about 0.5 mil to about 1.0mils.

In this three layer film embodiment, the thickness of the first outerlayer is preferably within the aforementioned range in order to obtaingood seal strength and acceptable film shrinkage. The thickness of thefirst core layer is preferably within the aforementioned range in orderto provide adequate oxygen barrier without detracting from toughnessproperties, but the upper limit of 0.45 mil is based upon the extent ofthe barrier protection which is required for the intended use for themultilayer film. The thickness of the second outer layer is preferablywithin the aforementioned range in order to make up the total filmthickness and to provide properties of abrasion resistance and punctureresistance.

As noted hereinabove, the present invention contemplates two alternateembodiments of multilayer film having four layers. The first alternateembodiment comprises a coextruded multilayer film containing a firstouter layer of a heat sealable thermoplastic polymer or copolymer; afirst core layer of an oxygen barrier material comprising vinylidenechloride-methyl acrylate copolymer; a second core layer comprising verylow density polyethylene, confined between said first outer layer andsaid first core layer, and bonded directly to said first core layerwithout the use of adhesives; and a second outer layer comprising verylow density polyethylene bonded directly to said first core layerwithout the use of adhesive. The second alternate embodiment comprises acoextruded multilayer film containing a first core layer of oxygenbarrier material comprising vinylidene chloride-methyl acrylatecopolymer; a heat sealable first outer layer comprising very low densitypolyethylene bonded directly to said first core layer without the use ofadhesives; a second outer layer comprising a thermoplastic polymer orcopolymer; and a second core layer comprising very low densitypolyethylene, confined between said first core layer and said secondouter layer, and bonded directly to said first core layer without theuse of adhesives.

In said first alternate embodiment of the present invention, thecoextruded multilayer film comprises a biaxially stretched thermoplasticfour layer film having a total thickness of from about 2.0 mils to about4.5 mils. The 2.0 mils lower limit is established by the total thicknessachieved in adding the lower limit of thickness for the four individuallayers. As previously noted, films having a thickness greater than about4.5 mils will produce bags which will be somewhat difficult to gatherand clip closed or they may be difficult to heat seal closed atefficient speeds on heat sealing vacuum packaging machines. The heatsealing first outer layer of thermoplastic polymer or copolymer willpreferably have a thickness of from about 1.1 mils to about 1.8 mils;the first core layer of vinylidene chloride-methyl acrylate copolymerwill preferably have a thickness of from about 0.25 mil to about 0.45mil; the second core layer of very low density polyethylene confinedbetween said first outer layer and said first core layer will preferablyhave a thickness of from about 0.35 mil to about 2.0 mils; and thesecond outer layer of very low density polyethylene will preferably havea thickness of from about 0.35 mil to about 2.0 mils, but morepreferably from about 0.5 mil to about 1.0 mil.

In the second alternate embodiment of the present invention, thecoextruded multilayer film comprises a biaxially stretched thermoplasticfour layer film having a total thickness of from about 2.0 mils to about4.5 mils. The 2.0 mils lower limit is established by the total thicknessachieved in adding the lower limit of thickness for the four individuallayers. As previously noted, films having a thickness greater than about4.5 mils will produce bags which will be somewhat difficult to gatherand clip closed or they may be difficult to heat seal closed atefficient speeds on heat sealing vacuum packaging machines. The heatsealing first outer layer of very low density polyethylene willpreferably have a thickness of from about 1.1 mils to about 1.8 mils;the first core layer of vinylidene chloride-methyl acrylate copolymerwill preferably have a thickness of from about 0.25 mil to about 0.45mil; the second core layer of very low density polyethylene confinedbetween said first core layer and the second outer layer will preferablyhave a thickness of from about 0.35 mil to about 2.0 mils; and thesecond outer layer of thermoplastic polymer or copolymer will preferablyhave a thickness of from about 0.35 mil to about 2.0 mils, but morepreferably from about 0.5 mil to about 1.0 mil.

In both alternate embodiments comprising coextruded four layer films,the thickness of the first outer layer is preferably within theaforementioned range in order to obtain good seal strength andacceptable film shrinkage. The thickness of the first core layer ispreferably within the aforementioned range in order to provide adequateoxygen barrier without detracting from toughness properties, but theupper limit of 0.45 mil is based upon the extent of the barrierprotection which is required for the intended use for the multilayerfilm. The thickness of the second core layer is preferably within theabove-indicated range in order to enhance the puncture resistance of thefilm without being too costly. The thickness of the second outer layeris preferably within the aforementioned range in order to make up thetotal film thickness and to provide properties of abrasion resistanceand puncture resistance.

In addition to providing good heat sealing when the film is fabricatedinto bags, multilayer films of the present invention have good shrinkproperties, good abrasion resistance and good toughness. Thus, thesefilms have utility in many packaging applications. However, in apreferred embodiment, these films are fabricated into bags for thepackaging of primal and subprimal meat cuts of fresh red meat, and forthe packaging of processed meats.

Such bags may be produced from the multilayer films of this invention byany suitable method, such as by heat sealing the side and/or bottomedges. For instance, if the film of this invention is produced in theform of a tubular film, bags can be produced therefrom by heat sealingone end of a length of the tubular film, or by sealing both ends of thetube end and then slitting one edge to form the bag mouth. If the filmof this invention is made in the form of flat sheets, bags can be formedtherefrom by sealing three edges of two superimposed sheets of film.When carrying out a heat sealing operation, the surfaces which are heatsealed to each other to form seams are the said first outer layers ofthe films of this invention. Thus, for example, when forming a bag byheat sealing one edge of a length of tubular film, the inner surface ofthe tube, i.e., the surface which will be heat sealed to itself, will bethe said first outer layer of the film. Accordingly, the first outerlayer of the film becomes the inner surface of the bag and the secondouter layer of the film becomes the outer surface of the bag.

The invention is further illustrated by Examples which are presentedhereinafter.

The resins which were used in making the multilayer films of theExamples are identifies as follows:

Ethylene Vinyl Acetate (EVA)

    ______________________________________                                        EVA-1:     DQDA 6832                                                                     Vinyl Acetate 11 Wt. %                                                        Melt Index 0.25 dg./min.                                                      Union Carbide Corporation;                                                    Danbury, CT                                                        EVA-2:     ELVAX 3135X                                                                   Vinyl Acetate 12 Wt. %                                                        Melt Index 0.35 dg./min.                                                      E. I. DuPont de Nemours & Co.,                                                Inc.; Wilmington, DE                                               EVA-3:     ENRON 3507C                                                                   Vinyl acetate 5 Wt. %                                                         Melt Index 0.25 dg./min.                                                      USI Chemicals Company                                                         Cincinnati, OH                                                     EVA-4:     A blend of EVA-2 and EVA-3                                                    75 Wt. % EVA-2, 25 Wt. % EVA-3                                                Contains about 9 Wt. % vinyl acetate                               ______________________________________                                    

Vinylidene Chloride-Vinyl Chloride Copolymer (VC-VDC)

    ______________________________________                                        VC-VDC:     Kureha F Resin                                                                Vinyl Chloride 29 Wt. %                                                       Molecular Weight 125,000                                                      Kureha Chemical Industry Co, Ltd.                                             Tokyo, Japan                                                      ______________________________________                                    

Vinylidene Chloride-Methyl Acrylate Copolymer (Ma-VDC)

    ______________________________________                                        MA-VDC:     XU 32023                                                                      Methyl Acrylate 8 Wt. %                                                       Molecular Weight 105,000                                                      Dow Chemical Co.; Midland, MI                                     ______________________________________                                    

Vinylidene Chloride Copolymer Blend

    ______________________________________                                                Blend:                                                                              75 Wt. % MA-VDC                                                               25 Wt. % VC-VDC                                                 ______________________________________                                    

Linear Low Density Polyethylene (LLDPE)

    ______________________________________                                        LLDPE-1: HS 7028                                                              Melt Index 1.0 dg./min.                                                       Density 0.918 gm./cc.                                                         Ethylene-Hexene Copolymer                                                     Union Carbide Corporation; Danbury,                                           CT                                                                            LLDPE-2: Dowlex 2045                                                          Melt Index 1.0 dg./min.                                                       Density 0.920 gm./cc.                                                         Ethylene-Octene Copolymer                                                     Dow Chemical Co.; Midland, MI                                                 ______________________________________                                    

Very Low Density Polyethylene (VLDPE)

    ______________________________________                                        VLDPE-1: DFDA 1137                                                            Melt Index 1.0 dg./min.                                                       Density 0.905 gm./cc.                                                         Ethylene-Butene Copolymer                                                     Union Carbide Corporation; Danbury,                                           CT                                                                            VLDPE-2: Dowlex 4002                                                          Melt Index 2.5 dg./min.                                                       Density 0.912 gm./cc.                                                         Etylene-Octene Copolymer                                                      Dow Chemical Co.; Midland, MI                                                 VLDPE-3: Dowlex 4001                                                          Melt Index 1.0 dg./min.                                                       Density 0.912 gm./cc.                                                         Ethylene-Octene Copolymer                                                     Dow Chemical Co.; Midland, MI                                                 ______________________________________                                    

VLDPE-2 and VLDPE-3 are commonly called "ultra low density polyethylene"or "ULDPE" by the Dow Chemical Company. Since these two resins havedensities which are below 0.915 gms./cc., for purposes of this inventionthey are very low density polyethylene. Therefore, we have identifiedthese resins as VLDPE-2 and VLDPE-3 for purposes of clarity andconsistency in the Examples which follow.

The properties of the resins and of the films produced therefrom may bedetermined by the following methods:

Density:

ASTM D-1505--Plaque is conditioned for one hour at 100° C. to approachequilibrium crystallinity--reported as gms/cm³.

Melt Index (MI):

ASTM D-1238--Condition E--measured at 190° C.

Haze:

ASTM D-1300, Procedure A.

Gloss:

ASTM D-523, 45° Angle.

Tensile Strength and Elongation At Break:

ASTM D-882, Procedure A

Shrinkage Values:

Values are obtained by measuring unrestrained shrink at 90° C. for fiveseconds. Four test specimens are cut from a given sample of the film tobe tested. The specimens are cut to 10 cm. in the machine direction by10 cm. in the transverse direction. Each specimen is completely immersedfor 5 seconds in a 90° C. water bath. The distance between the ends ofthe shrunken specimen is measured. The difference in the measureddistance for the shrunken specimen and the original 10 cm. is multipliedby ten to obtain the percent of shrinkage for the specimen. Theshrinkage for the four specimens is averaged for the MD shrinkage valueof the given film sample, and the shrinkage for the four specimens isaveraged for the TD shrinkage value.

Dynamic Puncture:

The dynamic puncture-impact test procedure is used to compare films fortheir resistance to bone puncture. It measures the energy required topuncture a test sample with a sharp triangular metal point made tosimulate a sharp bone end. A Dynamic Ball Burst Tester, Model No. 13-8,available from Testing Machines, Inc., Amityville, Long Island, NewYork, is used, and a 3/8 inch diameter conical tip is installed on thetester probe arm for use in this test procedure. The conical tip has theconfiguration of a right circular cone, and the angle between the coneaxis and an element of the conical surface at the vertex is about 65°.Six test specimens approximately 4 inches square are prepared, a sampleis placed in the sample holder, and the pendulum is released. Thepuncture energy reading is recorded. The test is repeated until 6samples have been evaluated. The results are calculated in cm-kg per milof film thickness and re averaged.

Hot Water Puncture:

Hot water puncture values are obtained by performing the hot waterpuncture test as follows. Water is heated to 90°±1° C. A 3/8 inch roundwooden dowel is sharpened on one end to a conical point. This sharpenedpoint has the configuration of a right circular cone, and the anglebetween the cone axis and an element of the conical surface at thevertex is about 60°. This sharp point is then rounded to a spherical tipof about 1/16 inch diameter. The wooden dowel is fastened to a woodenblock so that the rounded point projects 11/2 inches beyond the end ofthe wooden block. A specimen about 3 inches wide in the transversedirection (TD) and about ten inches long is cut from the test samplematerial. One end of the specimen is placed on the end of the woodenblock opposite the pointed dowel. The specimen is wrapped around the endof the sharpened dowel and back to the wooden block on the oppositeside, where it is secured. The film thickness in the area of contactwith the sharpened dowel is measured in order to assure that the filmspecimen thickness is truly representative of the given test samplematerial. The specimen and pointed dowel are quickly immersed into thehot water and a timer is started. The timer is stopped when the woodendowel point punctures the film specimen. The test procedure is repeatedfive more times with new 3 inch wide TD specimens from the given testsample material. The time required for penetration is recorded and thenaveraged for the six TD specimens.

Adhesion And Curl:

Although the properties of adhesion and curl are separate and distinctproperties of multilayer films, they are both determined by a singletest procedure. A coextruded multilayer film in tubular form having anominal length of about 2 feet is cut to provide a sample having astraight TD cut on one end and an arc TD cut on the other end. The twocuts simulate a straight bag mouth and an arcuate bag mouth. The filmsample is inserted into a hot air circulating oven set at 120° F. Afterseven days at 120° F., the sample is removed from the oven and it isinspected at both the straight and arcuate ends for curl anddelamination. In general, both ends of the tubular sample will exhibitthe same degree of curl and delamination. Delamination is generallyfound or not found at the interface between the oxygen barrier corelayer and the adjacent layer. The extent of delamination is reported as"adhesion", and the adhesion is categorized as being poor, fair, good orexcellent. Films having an adhesion of poor and fair are unacceptablefor packaging primal and subprimal meat cuts and processed meats. Curlis an indication of the ease or difficulty which will be experiencedwhen opening bags fabricated of the multilayer film of the given sample.An outward curl is an indication that the bags will be easily opened. Aninward curl is an indication of the degree of difficulty in opening abag. Curl is categorized as slightly inward, moderately inward, tightlyinward, slightly outward, moderately outward and tightly outward. Tubingsamples showing a tightly inward curl are deemed unacceptable asproviding bags which are too difficult to open. Tubing samples whichshow a moderately inward curl, a slight inward curl, or any degree ofoutward curl are deemed acceptable for the fabrication of bags.

The invention is now described further in the following Examples. In theExamples, all parts and percentages are by weight unless otherwiseindicated.

EXAMPLE 1

This Example illustrates the production of a first set of multilayerfilms under conventional coextrusion operating conditions.

A first series of coextrusion runs was made in order to produce threelayer films containing an oxygen barrier layer comprising the vinylidenechloride-vinyl chloride copolymer defined hereinabove as VC-VDC. Thefirst outer layer contained ethylene vinyl acetate copolymer identifiedhereinabove as EVA-1, and the EVA-1 was blended in eight of ten runswith other copolymers of ethylene identified hereinabove as LLDPE-1,VLDPE-1 and VLDPE-2. The second outer layer contained the blend ofethylene vinyl acetate copolymers which is identified hereinabove asEVA-4.

The melted resins were extruded from a multilayer annular die to producethe three layer film in a tubular form, wherein the first outer layerwas the inner layer of the tubular film. The tubular film was extrudedfrom the annular die to form a primary tube which was then biaxiallystretched in accordance with a conventional "double-bubble" techniquesimilar to that disclosed in U.S. Pat. No. 3,456,044 to Pahlke. Thebiaxially stretched multilayer film was flattened and then reeled forstorage and sample evaluation. The film produced by these runs was notirradiated.

The composition of each multilayer film produced in these runs is setforth as Samples No. A-1 through A-10 in Table 1. Note that all runsproduced films containing a core layer of VC-VDC and an outer layer ofEVA-4. The inner layer contained blends of EVA-1 with varying amount ofLLDPE-1, VLDPE-1, and VLDPE-2, except that Sample No. A-1 was 100 wt. %EVA-1 and Sample No. A-10 was 100 wt. % VLDPE-2. Sample No. A-1 is acontrol sample which is the equivalent of a film product which has beenin commercial use for many years in the packaging of primal andsubprimal meat cuts and processed meats, having been sold first by UnionCarbide Corporation of Danbury, CT, and now being sold by ViskaseCorporation of Chicago, IL, under the product identifications ofPERFLEX® 52 Bag and PERFLEX® 62 Bag.

The films produced in this series of runs had a total thickness of 2.4mils. The inner layer was 1.4 mils in thickness, the core layer ofVC-VDC was 0.3 mils thick, and the outer layer of EVA-4 was 0.7 milsthick.

EXAMPLE 2

This Example illustrates the production of a second set of multilayerfilms under conventional coextrusion operating conditions. These filmsinclude one embodiment of the present invention.

A second series of coextrusion runs was made in order to produce threelayer films containing an oxygen barrier layer comprising the blend ofvinylidene chloride-vinyl chloride copolymer with vinylidenechloride-methyl acrylate copolymer which is defined hereinabove. Theblend comprised 75 wt. % of the methyl acrylate copolymer and 25 wt. %of the vinyl chloride copolymer which are both defined hereinabove. Theprocess utilized was the same as that set forth in Example 1.

                  TABLE 1                                                         ______________________________________                                        Composition Of Coextruded Films                                               Sample No.                                                                             Inner Layer  Core Layer Outer Layer                                  ______________________________________                                        A-1      EVA-1        VC-VDC     EVA-4                                        A-2      15% LLDPE-1  VC-VDC     EVA-4                                                 85% EVA-1                                                            A-3      25% LLDPE-1  VC-VDC     EVA-4                                                 75% EVA-1                                                            A-4      15% VLDPE-1  V-VDC      EVA-4                                                 85% EVA-1                                                            A-5      25% VLDPE-1  VC-VDC     EVA-4                                                 75% EVA-1                                                            A-6      50% VLDPE-1  VC-VDC     EVA-4                                                 50% EVA-1                                                            A-7      15% VLDPE-2  VC-VDC     EVA-4                                                 85% EVA-1                                                            A-8      25% VLDPE-2  VC-VDC     EVA-4                                                 75% EVA-1                                                            A-9      50% VLDPE-2  VC-VDC     EVA-4                                                 50% EVA-1                                                            A-10     VLDPE-2      VC-VDC     EVA-4                                        B-1      50% LLDPE-2  Blend      50% LLDPE-2                                           50% EVA-1               50% EVA-4                                    B-2      VLDPE-3      Blend      VLDPE-3                                      B-3      EVA-1        Blend      EVA-4                                        ______________________________________                                         The coextruded biaxially stretched films were reeled for storage and     sample evaluation without irradiation of the films.

The composition of each multilayer film produced in these runs is setforth as Samples No. B-1 through B-3 in Table 1. Note that these runsproduced multilayer films containing an inner layer and an outer layerof the same composition in Samples No. B-1 and B-2. Sample No. B-1 hadinner and outer layers of a blend containing resin LLDPE-2, which isdefined hereinabove, and EVA-1 in equal amounts. Sample No. B-2 hadinner and outer layers of 100% VLDPE-3, which is defined hereinabove,and this Sample illustrates one embodiment of the present invention.Sample No. B-3 was a control sample similar to Sample No. A-1 in that itwas the equivalent of the prior art commercial product noted in Example1.

The films produced in this series of runs had a total thickness of 2.4mils. The inner layer had a thickness of 1.4 mils, the core layer was0.3 mil thick, and the outer layer was 0.7 mil thick.

EXAMPLE 3

This example summarizes the key physical properties of the unirradiatedreel stock multilayer films produced in the series of coextrusion runsdefined in Examples 1 and 2, in regard to film acceptability for use inthe packaging of primal and subprimal meat cuts and processed meats.

                                      TABLE 2                                     __________________________________________________________________________    Acceptability Of Coextruded                                                   Unirradiated Multilayer Films                                                 Sample                                                                            Layer Adhesion                                                                         Film Curl   Film Haze                                                                            Film Gloss                                                                           Acceptability                          No. Rating                                                                             Accept                                                                            Type    Accept                                                                            %  Accept                                                                            %  Accept                                                                            Of The Film                            __________________________________________________________________________    A-1 Good Yes Moderate Out                                                                          Yes 5.8                                                                              Yes 77 Yes Yes                                    A-2 Fair No  Tight In                                                                              No  7.2                                                                              No  66 No  No                                     A-3 Fair No  Tight In                                                                              No  9.2                                                                              No  60 No  No                                     A-4 Fair No  Moderate In                                                                           Yes 6.2                                                                              Yes 72 Yes No                                     A-5 Fair No  Tight In                                                                              No  7.5                                                                              No  68 No  No                                     A-6 Excellent                                                                          Yes Tight In                                                                              No  7.6                                                                              No  60 No  No                                     A-7 Poor No  Tight In                                                                              No  7.5                                                                              No  69 No  No                                     A-8 Poor No  Tight In                                                                              No  9.0                                                                              No  61 No  No                                     A-9 Poor No  Tight In                                                                              No  11.1                                                                             No  53 No  No                                     A-10                                                                              Poor No  Tight In                                                                              No  5.7                                                                              Yes 74 Yes No                                     B-1 Good Yes Slight In                                                                             Yes 15.1                                                                             No  41 No  No                                     B-2 Excellent                                                                          Yes Moderate In                                                                           Yes 1.9                                                                              Yes 87 Yes Yes                                    B-3 Good Yes Tight Out                                                                             Yes 2.3                                                                              Yes 86 Yes Yes                                    __________________________________________________________________________

Specimens of the unirradiated reel stock for each sample were evaluatedfor layer adhesion, film curl, haze and gloss in order to determine theacceptability of the various films in each of these categories. Theresults are presented in Table 2.

Layer adhesion is a visual evaluation of the degree of delamination, ifany, found at the interface between the core layer and the inner layer.Delamination may be exhibited by actual separation of the layers at theend cuts of the film specimen, or by the appearance of blisters at thesurface between the end cuts. The blisters are a sign of layerseparation in the body of the film between the end cuts. Only the priorart control Samples No. A-1 and B-3, and new film Samples No. A-6, B-1and B-2 showed acceptable adhesion.

Note that the prior art films of Samples No. A-1 and B-3, which containinner and outer layers of EVA, showed an adhesion rating of Good, whilethe invention film of Sample No. B-2, which contained inner and outerlayers of VLDPE had an adhesion rating of Excellent. Thus, the filmembodiment of this invention showed an adhesion rating which wasimproved over the adhesion rating of the prior art films. This iscontrary to the teachings of Ferguson et.al. U.S. Pat. No. 4,640,856which states that VLDPE does not adhere as well as EVA does.

The evaluation of film curl showed that only the prior art commercialtype films of Samples No. A-1 and B-3 had an outward curl, which is themost desired type of curl for ease in opening the mouth of a bag made ofthe multilayer film. For the experimental multilayer films which wereevaluated, only samples A-4, B-1 and B-2 had an acceptable curl. SampleB-1 had a slight inward curl, and Samples A-4 and B-2 had a moderateinward curl. All other samples had an unacceptable tight inward curl.

The differences in the direction of curl, inward or outward, and in thedegree of the curl are caused by the differences that exist between theinner layer and the outer layer. When LLDPE or VLDPE is blended into theEVA-1 of the inner layer, the balance between the inner layer and theouter layer becomes distorted and the film exhibits the unacceptabletight inward curl of Samples No. A-2, A-3 and A-5 through A-10.

Curl also has an influence on adhesion since it can impose stresses onthe layers which may cause delamination. This is the probable reason whyonly those samples which had both acceptable adhesion and acceptablecurl are Samples No. B-1 and B-2, where the inner and outer layers bothhad the same composition.

The data in Table 2 demonstrates that a multilayer film containing LLDPEor VLDPE in the inner layer must also have the same composition in theouter layer in order to assure that the film will have acceptableadhesion and acceptable curl.

The data in Table 2 also shows that films which contain a blend of EVA-1with LLDPE or VLDPE exhibit unacceptable optical characteristics. Theonly exception is Sample No. A-4 which showed acceptable haze andacceptable gloss, but this sample had a low VLDPE content of 15 wt. % inthe inner layer. Also, this sample had an unacceptable adhesion. SampleNo. A-10 containing 100 wt. % VLDPE-2 in the inner layer and Sample No.B-2 containing 100 wt. % VLDPE-3 in both the inner and outer layers alsohad acceptable haze and acceptable gloss. (As noted hereinabove, hazevalue must not exceed 6.5% and gloss value must not be below 70% for afilm to be acceptable).

In order for a multilayer film to be acceptable for use in the packagingof fresh red meat cuts and processed meats, the film must be acceptablein all four catagories of adhesion, curl, haze and gloss, and the datashow that only three of the film samples meet this standard. They arethe prior art Samples No. A-1 and B-3, and the new Sample No. B-2 whichis an embodiment of our invention.

Sample No. B-2 is also noteworthy, not only for being the only new filmmeeting all standards for film acceptability, but also because itexhibits the best adhesion, the best curl, the best haze, and the bestgloss for all of the new film samples.

The data in Table 2 also indicate that excellent adhesion and acceptablecurl are obtained if the core layer is a blend of 75 wt. % MA-VDC and 25wt. % VC-VDC with 100 wt. % VLDPE in both the inner and outer layers.This blend demonstrates that the core layer may contain a substantialquantity of VC-VDC in the MA-VDC blend without adversely effecting thesuperior physical characteristics of the multilayer films of thisinvention. A core layer of 100 wt. % MA-VDC will also give acceptableadhesion and curl with inner and outer layers of 100 wt. % VLDPE.

EXAMPLE 4

This Example illustrates that the embodiment of our invention which isrepresented by Sample No. B-2, not only has acceptable adhesion, curl,haze and gloss, but that it also has other properties which make itsuitable for the packaging of primal and subprimal meat cuts andprocessed meats.

The inventive multilayer film Sample No. B-2 and the prior artmultilayer film of Samples No. A-1 and B-3 were evaluated for tensilestrength, elongation at break, shrinkage, hot water puncture, anddynamic puncture. The test results are given in Table 3.

                  TABLE 3                                                         ______________________________________                                        Properties Of Coextruded                                                      Unirradiated Multilayer Films                                                              Sample  Sample    Sample                                                      No. A-1 No. B-2   No. B-3                                        ______________________________________                                        Tensile Strength, psi.                                                                       7,100/    11,900/   7,500/                                     MD/TD          8,700     12,400    8,800                                      Elongation @ Break, %                                                                        215/150   240/170   215/170                                    MD/TD                                                                         Shrinkage @ 90° C.                                                                    35/52     21/32     37/53                                      MD/TD                                                                         Hot Water Puncture, sec.                                                                     29        120+      24                                         @ 90° C.                                                               Dynamic Puncture,                                                                            2.0       2.7       2.1                                        cm.-kg./mil                                                                   ______________________________________                                    

The inventive multilayer film Sample No. B-2 had tensile strength, hotwater puncture, and dynamic puncture values which were improved over thesamples of prior art film. The shrinkage values were less than thevalues for the prior art film, but the shrinkage remained at anacceptable level. Elongation at break was about the same for all threesamples, and was acceptable.

Although certain embodiments of this invention have been described indetail, it is contemplated that modifications thereof may be made andsome preferred features may be employed without others, all within thespirit and scope of the broad invention. For example, although SampleB-2 had the same very low density polyethylene on both sides of the corelayer of vinylidene chloride-methyl acrylate copolymer, it is possiblefor two different VLDPE resins to be used, provided that their physicalcharacteristics must not be so different that unacceptable curl orunacceptable adhesion results. Additionally, it is contemplated that theVLDPE of the first layer, or of the second layer, or of both the firstand the second layers, may comprise a blend of VLDPE with one or moreother polymers or copolymers, provided that such blends must not causedegradation of adhesion, curl, haze or gloss to such an extent that themultilayer film is rendered unacceptable. Further, those skilled in theart will recognize that the multilayer films of this invention maycontain conventional additives such as pigments, antiblock agents, slipagents, and the like.

The present invention is now set forth with particularity in the claimswhich follow. As used in the claims, the term "polymer" includeshomopolymers and copolymers.

The invention claimed;
 1. A process of producing a thermoplastic, heatshrinkable, multilayer film suitable for use in packaging fresh redmeats and processed meats, which comprises coextruding a core layercomprising a vinylidene chloride-methyl acrylate copolymer having avinylidene chloride content of from about 85 to about 95 weight percentand a methyl acrylate content of from about 5 to about 15 weight percentall based on the weight of said copolymer, a first layer adhereddirectly to one side of said core layer without adhesive materialtherebetween and comprising very low density polyethylene of density notgreater than about 0.915 grams per cubic centimeter, and a second layeradhered directly to the other side of said core layer without adhesivematerial therebetween and comprising very low density polyethylene ofdensity not greater than about 0.915 grams per cubic centimeter.
 2. Aprocess according to claim 1 wherein said first layer and said secondlayer are identical.
 3. A process according to claim 2 wherein saiddensity is in the range of from about 0.86 to about 0.915 grams percubic centimeter.
 4. A process according to claim 1 wherein said verylow density polyethylene of said first layer and said very low densitypolyethylene of said second layer have a melt index in the range of fromabout 0.5 to about 2.5 decigrams per minute.
 5. A process according toclaim 1 wherein said very low density polyethylene of said first layerand said very low density polyethylene of said second layer have adensity of about 0.912 grams per cubic centimeter and a melt index ofabout 1.0 decigrams per minute.
 6. A process according to claim 1wherein said very low density polyethylene of said first layer and saidvery low density polyethylene of said second layer are copolymers ofethylene and octene-1.
 7. A process according to claim 1 wherein saidcore layer comprises vinylidene chloride-methyl acrylate copolymerblended with vinylidene chloride-vinyl chloride copolymer.
 8. A processaccording to claim 7 wherein said blend comprises about 75 weightpercent vinylidene chloride-methyl acrylate copolymer and about 25weight percent vinylidene chloride-vinyl chloride copolymer.
 9. Aprocess according to claim 1 wherein said first layer is a second corelayer confined between said first core layer of vinylidenechloride-methyl acrylate copolymer and a coextruded first outer layercomprising a heat sealable thermoplastic polymer.
 10. A processaccording to claim 9 wherein said first outer layer comprises a heatsealable thermoplastic polymer selected from the group consisting of anionomer, an ethylene vinyl acetate copolymer, an ethylene-propylenecopolymer, and a propylene blended with another polymer.
 11. A processaccording to claim 1 wherein said second layer is a second core layerconfined between said first core layer of vinylidene chloride-methylacrylate copolymer and a coextruded second outer layer comprising athermoplastic polymer.
 12. A process according to claim 11 wherein saidsecond outer layer comprises a blend of high density polyethylene withlinear low density polyethylene or very low density polyethylene.
 13. Aprocess according to claim 12 wherein the amount of high densitypolyethylene in said blend is not greater than about 30 weight percentof the blend.
 14. A process according to claim 1 wherein saidcoextrusion is by means of a slot die and said multilayer film is insheet form.
 15. A process according to claim 1 wherein said coextrusionis by means of an annular die and said multilayer film is in tubularform.
 16. A process according to any one of claims 1 through 15 whereinsaid coextruded multilayer film is biaxially stretched.
 17. A processaccording to claim 16 including the step of fabricating said multilayerfilm into a bag.